1. Field of the Invention
The present invention relates to a radio-frequency amplifier. In particular, the invention relates to a radio-frequency amplifier in a transmission power amplifier for a CDMA modulation type portable terminal.
2. Background Art
In recent years, small information terminals (hereinafter referred to as xe2x80x9cportable terminalsxe2x80x9d) such as cellular phones and PDAs have spread. In portable terminals, amplifiers having high power consumption efficiency are now required. Among multiple access schemes employed by cellular phones is a code division multiple access (CDMA) modulation scheme that is based on a spread spectrum technique. In transmission power amplifiers for CDMA modulation type portable terminals, not only the power load efficiency at the time of high output power but also the efficiency at the time of low output power is important because a power control is performed during a communication. Therefore, conventionally, in such amplifiers, switching is made between bias settings for high output power and low output power.
FIG. 4 shows a conventional power amplifier 60 with a bias circuit having a bias setting switching function. In FIG. 4, reference numeral 1 denotes a radio-frequency (RF) transistor; 7, a terminal of a collector voltage source for the RF transistor 1; 3, a bias circuit unit for supplying a base bias to the RF transistor 1; and 2, a bias switching circuit unit for switching between bias settings. In the bias circuit unit 3, reference numeral 10 denotes a transistor Q1; 6, a terminal of a bias circuit voltage source Vcb that is connected to the collector of the transistor Q1 (10); 8, a resistor R1 that is provided between the emitter of the transistor Q1 (10) and the ground GND; 5, a terminal of a reference voltage Vref; 9, a resistor R2 that is provided between the terminal 5 of the reference voltage Vref and the base of the transistor Q1 (10); and 23 and 24, transistors that are provided between the base of the transistor Q1 (10) and the ground GND. In the bias switching circuit unit 2, reference numeral 11 denotes a transistor Q2; 21, a resistor that is provided between the collector of the transistor Q2 (11) and the base of the RF transistor 1; 4, a bias switching MOD terminal; and 22, a resistor that is provided between the base of the transistor Q2 (11) and the bias switching MOD terminal 4.
Next, an operation of the conventional amplifier 60 will be described with reference to FIG. 4. When the amplifier 60 is in high output power operation, the bias switching MOD terminal 4 receives an off-voltage (0 V) and hence the transistor Q2 (11) of the bias switching circuit unit 2 is turned off. As a result, the voltage that is output from the bias circuit unit 3 is supplied to the RF transistor 1 without modification. The base bias that is supplied to the RF transistor 1 at this time is set so that an output power distortion characteristic that is required for the amplifier 60 in high output power operation is satisfied with a sufficient margin.
On the other hand, when the amplifier 60 is in low output power operation, the bias switching MOD terminal 4 is given an on-voltage (3 V) and hence the transistor Q2 (11) of the bias switching circuit unit 2 is turned on. As a result, a current flows through the bias switching circuit unit 2 and the base bias of the RF transistor 1 is lowered. When the amplifier 60 is in low output power operation, even if the base bias of the RF transistor 1 is lowered, the amplifier 60 operates in a linear range and hence its distortion characteristic should be satisfactory. The above-described bias switching function can increase efficiency at the time of low output power when the amplifier 60 operates in a linear range.
As described above, in the conventional bias circuit having the function of switching the base bias of the RF transistor 1, the base bias that is set by the bias circuit unit 3 is subjected to switching of the bias switching circuit unit 2. This causes a problem that a variation in the idle current of the RF transistor 1 strongly depends on a variation in the bias voltage that is output from the bias circuit unit 3. When the amplifier 60 is in low output power operation, its efficiency strongly depends on the idle current of the RF transistor 1, to increase the efficiency of the amplifier 60 and thereby it is necessary to accurately set the bias that is output from the bias circuit unit 3. When the amplifier 60 is in low output power operation, a current of about 100 mA flows through it and hence a current of about 5 mA is consumed in the bias circuit unit 3. There is a problem that this current greatly influences the efficiency of the amplifier 60.
The present invention has been achieved to solve the above problems and an object of the invention is therefore to provide a radio-frequency amplifier capable of increasing its efficiency by decreasing a variation in the idle current of an RF transistor.
According to one aspect of the present invention, a radio-frequency amplifier is provided for amplifying a radio-frequency signal. The radio-frequency amplifier comprises a radio-frequency transistor, a reference voltage supply unit for supplying a first base bias from a reference voltage source to a base of the radio-frequency transistor via a first resistor, and a bias circuit unit for outputting a second base bias to the base of the radio-frequency transistor. The bias circuit unit comprises a switching terminal that provides a signal that serves to switch outputting the second base bias or not. The radio-frequency transistor performs a constant voltage operation and the radio-frequency amplifier produces a high output power when the bias circuit unit outputs the second base bias. The radio-frequency transistor performs a constant current operation and the radio-frequency amplifier produces a low output power when the bias circuit unit does not output the second base bias.
Other and further objects, features and advantages of the invention will appear more fully from the following description.